A LNT is an exhaust after-treatment device for lean burn engines. The LNT has to be purged periodically to release and convert the oxides of nitrogen (NOX) stored in the LNT during lean operation of the engine. To accomplish the purge, the engine has to be operated at an air-to-fuel ratio that is rich of stoichiometric. As a result of the rich operation, substantial amounts of carbon monoxide (CO) and hydrocarbons (HC) are generated to convert the stored NOX. Typically, the purge mode is activated on the basis of estimated LNT loading. That is, when the estimated mass of NOX stored in the LNT exceeds a predetermined threshold, a transition to the purge mode is initiated. The rich operation will be continued for several seconds until the LNT is emptied of the stored NOX, whereupon the purge mode is terminated and the normal lean operation of the engine is resumed. The ending of the purge is normally initiated by a transition in the reading of an Oxygen sensor such as a HEGO sensor or NOx sensor located downstream of the LNT or is based on a modelled prediction of the LNT state. Since the engine has to be operated rich of stoichiometric during the purge operation, purging will have a significant negative effect on fuel economy compared to the fuel economy advantage of lean operation and so it is desirable to optimize the timing of the purge in order to reduce the loss in fuel economy.
It is well known that the regeneration of a LNT aftertreatment device can be more efficiently carried out when the vehicle is being operated in a particular manner such as at a high load because the fuel penalty is then reduced.
It is further known that when a LNT fills with NOx there is an increased risk of NOx slippage from the LNT. NOx slippage from an LNT will result in increased emissions from the vehicle and so is undesirable.
It has been proposed in, for example U.S. Pat. No. 7,685,813 to predict the future usage of a vehicle by using a navigation system. The navigation system, such as a GPS system, is used to predict the future route from which the expected usage of the vehicle while it is traversing that route can be derived which can be useful in deciding when good conditions exist for purging the LNT. However, the use of a navigation system to predict the future usage or operation of a vehicle cannot be used in all cases because not all vehicles are fitted with such navigation equipment.
The inventors have realized that by using knowledge of the future operation of a vehicle the regeneration of the LNT can be scheduled to occur when the engine of the vehicle is expected to be operating in a predefined manner that is suitable for the purging of NOx from the LNT. The inventors have further realized that by using knowledge of the future operation of a vehicle it is possible to purge the LNT before the vehicle is operated in a manner likely to produce NOx slippage from the LNT thereby eliminating or significantly reducing the risk of NOx slippage from the LNT.
It is an object of the invention to provide a method of pre-emptively regenerating a lean NOx trap arranged to receive exhaust gas from a lean burn engine of a vehicle that is cost effective to implement and reduces the risk of NOx slippage occurring.
According to a first aspect of the invention there is provided a method of pre-emptively regenerating a lean NOx trap arranged to receive exhaust gas from a lean burn engine of a vehicle the method comprising using a prediction of future operation of the vehicle to estimate the probability of NOx slippage from the lean NOx trap during the current drive cycle, and, if NOx slippage is expected to occur in the current drive cycle, use the prediction of future operation of the vehicle to infer whether there are any forthcoming opportunities to regenerate the lean NOx trap in a favorable manner before the slippage is predicted to occur and, if such favorable opportunities exist, schedule a regeneration of the lean NOx trap for the next favorable opportunity.
The probability of whether NOx slippage is likely to occur may be based upon the current level of NOx stored in the lean NOx trap and at least one engine operational factor.
The probability of whether NOx slippage is likely to occur may be based upon at least two engine operational factors.
One engine operational factor may be an expected drop in air/fuel ratio.
One engine operational factor may be an expected increase in exhaust gas temperature.
One engine operational factor may be an expected significant change in exhaust gas mass flow to the lean NOx trap.
Regeneration of the lean NOX trap in a favorable manner may be scheduled to occur when the prediction indicates that the engine is expected to be operating with a low air/fuel ratio.
The engine may be expected to be operating with a low air/fuel ratio when the load on the engine is high.
The engine may be expected to be operating with a low air/fuel ratio when the load on the engine is high and the speed of the engine is low.
The prediction of future operation of the vehicle may be obtained using a common route predictor method.
The common route predictor method may comprise the steps of comparing a current pattern of operation with stored common patterns of operation and, if the current pattern of operation conforms to a particular one of the stored common patterns of operation, using the particular one of the stored common patterns of operation for predicting the future operation of the vehicle during the current trip.
The common route predictor method may further comprise monitoring the operation of the vehicle for each trip conducted by the user, storing data collected from the monitoring and establishing the common patterns of operation from the stored data.
According to a second aspect of the invention there is provided a vehicle having a lean burn engine, a lean NOx trap arranged to receive exhaust gas from the engine and an electronic processing system to control regeneration of the lean NOx trap wherein the electronic processing system is operable to use a prediction of future operation of the vehicle to estimate the probability of NOx slippage from the lean NOx trap during the current drive cycle, and, if NOx slippage is expected to occur in the current drive cycle, use the prediction of future operation of the vehicle to infer whether there are any forthcoming opportunities to regenerate the lean NOx trap in a favorable manner before the slippage is predicted to occur and, if such favorable opportunities exist, schedule a regeneration of the lean NOx trap for the next favorable opportunity.
The electronic processing system may be further operable to produce the prediction of future operation of the vehicle.
The prediction of future operation of the vehicle may be obtained using a common route predictor method.
The common route predictor method may be performed using the electronic processing system and may comprise the steps of comparing a current pattern of operation with stored common patterns of operation and, if the current pattern of operation conforms to a particular one of the stored common patterns of operation, using the particular one of the stored common patterns of operation for predicting the future operation of the vehicle during the current trip.
The common route predictor method may further comprise monitoring the operation of the vehicle for each trip conducted by the user, storing data collected from the monitoring and establishing the common patterns of operation from the stored data.
The invention will now be described by way of example with reference to the accompanying drawings.